The servlet and all filters involved in an async request must have async
support enabled using the Servlet API or by adding an
<async-support>true</async-support> element to servlet and filter
declarations in web.xml

isAsyncStarted

Whether the request is in async mode following a call to AsyncWebRequest.startAsync().
Returns "false" if asynchronous processing never started, has completed,
or the request was dispatched for further processing.